CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 19 



enlargement it was possible to follow from stage to stage the change in volume and 

 in form of the constituent tissue masses. Some of the photographs are reproduced 

 on Plates I and II. In drawing conclusions from such photographs account was 

 taken of the fact that the technique of preparing the serial sections introduces an 

 element of uncertainty in that some embryos in the process of embedding shrink 

 more than others. This is particularly so in human embryos, where there is nec- 

 essarily some difference in the freshness of the material at the time it is obtained. 

 Furthermore, even in the same embryo some tissues are affected by the technique 

 more than others. Due allowance was made for these factors. 



In order to determine the form and relations of the periotic-tissue spaces, wax- 

 plate models of the membranous labyrinth and the surrounding spaces were recon- 

 structed after the Born method. Advantage was taken of the improvements in 

 the method devised by Lewis (1915). The serial sections were photographed at 

 a suitable enlargement on bromide paper. By means of a preliminary model of 

 the membranous labyrinth the necessary reconstruction lines were established and 

 transferred to the bromide prints. From these prints the membranous labyrinth 

 and the periotic spaces were then traced on wax plates. After cutting out from 

 the plates the areas corresponding to these structures the plates were piled and the 

 resultant cavity was filled with plaster of paris. When the wax was finally melted 

 off there remained a permanent plaster cast of the objects desired at a definite 

 enlargement. Views of these models are shown on plate 4. 



In outlining the periotic spaces it was found necessary to adopt an arbitrary 

 rule as to how much should be included in the model. The smaller spaces of the 

 reticulum surrounding the main cavities can be seen coalescing to form larger 

 spaces, and these in turn coalesce with the main cavity as it advances into new 

 territory. There is thus a considerable range in the size and completeness of the 

 spaces in any one section. The main spaces and the larger adjacent ones that 

 communicate with them are outlined by a membrane-like border. This character- 

 istic was adopted as the guide in determining which spaces to admit into the model ; 

 only those possessing a more or less complete border of this kind were included. 



DEVELOPMENT OF THE CARTILAGINOUS CAPSULE OF THE EAR. 



When the present study was undertaken the writer's interest concerned more 

 particularly the process of conversion of the periotic reticular tissue into the walled- 

 off spaces that constitute the scala tympani, the scala vestibuli, and vestibular 

 cistern. It was soon found, however, that this could not be satisfactorily treated 

 without a consideration of the earlier history of this tissue and its relation to the 

 surrounding cartilaginous capsule. Therefore, a preliminary survey was made of 

 the earlier histogenetic processes of all the mesenchymal elements of the inner ear. 

 The character of these processes will form the subject-matter of the first part of 

 this paper. In brief, they include: (1) the original condensation of the mesenchyme 

 around the otic vesicle; (2) the subsequent differentiation of the condensed mesen- 

 chyme into precartilage on the one hand and periotic reticular tissue on the other; 

 (3) the differentiation of true cartilage and its manner of growth and alteration in 



